Gas-handling apparatus and method

ABSTRACT

We disclose gas-handling apparatus comprising a primary evaporator, first conduit connections for establishing a flow path for a compressed gas and the like through said evaporator, second conduit connections for establishing a flow path of a refrigerant fluid through said primary evaporator in heat exchanging relation with said gas path, a refrigerant unit coupled to said second conduit connections, and an auxiliary evaporator coupled between said refrigerating unit and said primary evaporator.

United States Patent 1,853,236 4/1932 Shadle inventors Alden '1. BluxhnmBrldgcvilk. Pm: Harry (J. Fischer. Royal Oak. Milli. Appl. No. 784,597Filed Dec. 18. 1968 Patented Aug. 3. 1971 Assignee Kellogg American.inc.

Oakmont, Pa. by said Harry C. Fiscimr GAS-HANDLING APPARATUS AND METMQD16 Claims. 7 Drawing Figs.

US. Cl 62/93. 62/113. 62/85. 62/513. 62/272 Int. Cl F256 17/116 Field ofSearch 62/85. 93. 513. l 13, 272

Rclercnces (31ml UNITED STATES PATENTS 1.969.227 15/1933 62/2722.051.971 6/1926 62/513 2.120.764 6/1938 62/573 2.477.772 6/1949 62/932.766.254 6/1966 Mallmfi.......... 62/513 2.841.965 7/1959 Ethiirington62/113 3.041.842 7/1962 Hiiirmfliifi 62/93 3.050.954 13/1962 62/933.359.763 12/1967 Fimilm 62/93 Primary Examiner william .1. Wyn Anameyman .1. Smith AHSTWWT: W6 disnlosm gwhnnclling apparatus comprising aprimary avapnrator. first caonciuit mnneqtiuns for establish ing a flowpath fur a rmmprmmmi gm; and this like through said mvapormm. amendwncluii qonnmtiuna for establishing a flow path of a refrigqmm fluidihruugh said primary evaporator in heat axchnnging ralation with saidgas path. a refrigerant unit coupled in said :mcond qoncluit aonnemiona.and an auxilinry evapmator euuplqcl bmwman said mfrigarming; unit andsaid primary cwnpormm.

GAS-HANDLING APPARATUS AND METHOD The present invention relates to meansand methods of handling compressed gases and more particularly to dryersof the refrigeration type for compressed air and other gases.

Although our invention is described herein primarily with reference .tocompressed air handling equipment, it will be obvious as thisdescription proceeds that our invention can be used with other gases orcombinations of gases.

A variety of equipment for cleaning and/or drying compressed airandother gasesis known. Widely ranging operating conditions have made suchequipment difficult to control with the desired .stability. For example,in known refrigeration drying systems, stable control is difficult wherethe drying function is variable, i.e., when the compressed air or othergas is supplied .in differingdegrees of wetness. Simple pressure andtemperature control means usually are not adequate for the requireddegree of control. The separated moisture may freeze .in parts of thesystem and render it inoperative.

The operational difficulties of conventional equipment for handlingcompressed air and other gases are further aggravated where thecompressed gas must be supplied at widely varying conditions of pressureand temperature. Conventional equipment hasbeen unable to supply,without complex controls and other complicated components, a consistentdewpoint dryness under these varyingconditions, which are furthercomplicated by variations in ambient air temperatures. For example, inindustrial applications, compressed air handlingequipment may be usedfor hot wet incoming air varying from p.s.i.g. to 1,000 p.s.i.g. at aninlet temperature of 130 F. or higher. Ambient temperatures may varybetween 40 F. and i10 F. Under these conditions rather complicatedconventional. equipment is required to provide cooled, com pressed gas.with an acceptable and consistent dewpoint dryness. insofar aswe areaware there is no available equipment which is capable of continuousoperation and wherein the compressed air drying function can bevaried atwill between zero load and full load while maintaining a consistentdewpoint dryness.

in the past, a number of air handling systems have been proposed,typified by the U.S. patents to Coblentz, U.S. Pat. No. 2,632,315;Ritter, U.S. Pat. No. 2,5l3,679; Meckler, U.S. Pat. No. 3,102,399;Shipman, U.S. Pat. No. 2,257,983; Cook et al.. U.S. Pat. No. 2,1 l9,20l;Schweller, U.S. Pat. No. 2,692,481; Newton, U.S. Pat. No. 2,367,305;Carrier, U.S. Pat. No. 2,154,263; Kohut, U.S. Pat. No. 2,835,476; andLund, U.S. Pat. No. 2,451,682,-al| of which suffer from theaforementioned defects. Most of these prior systems utilize variousarrangements of evaporators and regenerative heat exchangers for theincoming airor other gas which is being cooled, cleaned and/ordehumidified. However, one of these systems is capable of an output ofcompressed air having consistent dewpoint dryness over a wide range ofoperating or am bient conditions. in particular, none of thesereferences em ploys primary and auxiliaryevaporators together with othernovel features of our invention, which are described below. As mentionedabove, freezing is likely to occur under certain load conditions.

Freezing of moisture in such equipment has been prevented in the past bybrute force techniques involving for example a capillary expansion valvecoupled with a hot-gas bypass valve.

The hot-gas bypass valve is particularly subject to erosion of thebypassed gases asa result of scouring and cavitation. The bypass valveis complicated in construction and difficult to manufacture and repair.The capillary expansion valve cannot be regulated properly, save byreplacing with a differently sized valve.

Thermostatic expansion valves also are commonly used for controlling gashandling and drying equipment. These valves are rather complex and aresubject to hunting. Moreover, the control function provided bythermostatic expansion valves is confined to a very narrow range ofoperating conditions.

3 When the design limits of these valves are exceeded, even slightly,the control function is largely lost.

We overcome these deficiencies of the prior art by providing acontinuously operable refrigeration drying system capable of consistentdryness in output air irrespective of variable operating and ambientconditions. Our invention'engenders stability and controllabilitythrough the use of a unique arrangement of primary and auxiliaryevaporators. These characteristics are further enhanced by use of anautomatic expansion valve or the like, in conjunction with ourevaporative system. The refrigeration system of our invention is capableof continuous noncycling operation, and the actual compressed air dryingfunction thereof can be selectively varied from zero to full load, whileproducing a consistent dewpoint dryness of exit air. Although wedescribe our novel gas-handling equipment primarily in terms of removingor controlling the moisture content of compressed air, the compressedair or other gas is further cleaned by removal of oil and otherentrained foreign matter along with condensed moisture. Compressed gascan be dried or otherwise cleaned by removal of other condensibles orentrained matter with our novel equip ment.

Our invention also contemplates the use of an automatic" expansionvalve, which in this example is pressure sensitive. This type ofexpansion valve is of known construction but is provided with aninternal pressure transmitting aperture where the valve is sensitized toline pressure. Alternatively, the valve can be provided instead with ableed line where it is desired to sensitize he valve to pressure changesin an external conduit.

in conventional refrigeration drying systems, a pressuresensitiveexpansion valve exhibits relatively poor responses to load changes inthe system. We have found, however, that a pressure sensitive expansionvalve exhibits an admirable control characteristic and readily adjustsitself to the compressor load, when the compressor is run constantly andat the same speed. it is also desirable that the refrigerant compressorbe provided with adequate size so that it is capable of forcing theautomatic" expansion valve further open under certain conditions. As aresult the expansion valve responds quickly to changes in load demandupon the equipment. In this respect, the expansion valve of theinvention adjusts readily to load changes at the refrigerant evaporatorand compressor. The pressure sensitive means of the expansion valve inconjunc tion with the continuously operated compressor controls theexpansion valve such that the flow of refrigerant varies through mainand auxiliary evaporators of the invention with variation in load on theload or air side of the main evaporator. This diverts the evaporation orcondensation from a conventional condenser of the refrigeration unit tothe aforementioned auxiliary evaporator and/or diverts evaporation fromthe main evaporator to the auxiliary evaporator, depending upon theamount and direction of load change. For this purpose, thepressure-sensitive means is coupled to the refrigerant path adjacent itsoutlet from the main evaporator.

With this arrangement of our invention, freezing of the removed moistureis obviated. Expansion of the refrigerant fluid is closely controlledwith respect to load changes, irrespective of variable input and ambientconditions. The arrangement is not sensitive to narrow designlimitations nor is it subject to hunting.

We accomplish these desirable results by providing gashandling apparatuscomprising a primary evaporator, first conduit connections forestablishing a flow path for a compressed gas or the like through saidevaporator, second conduit connec tions for establishing a flow path ofa refrigerant fluid through said primary evaporator in heat exchangingrelation with said gas path, a refrigerating unit coupled to said secondconduit connections, and an auxiliary evaporator coupled between saidrefrigerating unit and said primary evaporator.

We also desirably provide similar gashandling apparatus wherein saidauxiliary evaporator is coupled regeneratively to said secpnd conduitconnections.

We also desirably provide similar gas-handling apparatus wherein asecond heat exchanger for said gas is coupled regeneratively to saidfirst-mentioned conduit connections.

We also desirably provide similar gas-handling apparatus 'wherein anexpansion valve and a third conduit connection are coupled between saidprimary evaporator and said refrigerating unit in bypassing relation tosaid auxiliary evaporator. 1

We also desirably provide a heat-exchanging structure for an evaporatorand the like, said structure comprising a plurality of sheath tubes, aplurality of inner tubes inserted respectively and spacediy within saidsheath tubes, and flow means for connecting one group of the group ofsheath tubes and the group of coaxial tubes in series and for connectingat least some of the other group of tubes in parallel.

We also desirably provide a method for operating refrigeratinggas-handling apparatus having a compressor, condenser, evaporator, andexpansion valve, said valve being mounted for expansion of a refrigerantfluid into said evaporator, said method comprising the steps ofoperating said compressor continuously, preventing complete closure ofsaid valve, sensitizing opening and closing movements of said valve topressure fluctuations at a given point in said apparatus, and sizingsaid compressor for operation at constant speed and for forcing saidvalve furtheropen in response to load changes in a given direction.

During the foregoing discussion, various objects, features andadvantages of the invention have been set forth. These and otherobjects, features and advantages of the invention together withstructural details thereof will be elaborated upon during theforthcoming description of presently preferred embodiments of theinvention and presently preferred methods of practicing the same.

in the accompanying drawings we have shown certain presently preferredembodiments of the invention and have illustrated presently preferredmethods of practicing the same, wherein:

FIG. 1 is a schematic fluid circuit of one arrangement of our compressedgas handling equipment;

FIG. 2 is top plan view, with parts broken away and other parts removed,of a packaged equipment assembly arranged in accordance with FIG. 1;

FIG. 3 is a front elevational view of the apparatus as shown in F IG. 2;

FIG. 4 is a left side elevational view of the apparatus as shown in FIG.3;

FIG. 5 is a cross-sectional view of the primary evaporator shown in FIG.2 and taken along reference line V-V thereof;

FIG. 6 is another cross-sectional view of the evaporator of FIG. 2 andtaken along reference line Vl-Vl thereof; and

FIG. 7 is still another cross-sectional view of the evaporator of FIG. 2taken along reference line Vll-Vll thereof.

Referring now more particularly to the drawings, an exemplarygas-handling equipment 10 of our invention comprises a refrigeration andcondensing unit 12 of known construction, regenerative heat exchanger14, primary evaporator 16 and auxiliary evaporator 18. The receiver 20of the refrigeration and condensing unit 12 is connected through conduit22 to expansion valve 24 and thence to the main evaporator 16 where therefrigerant fluid is vaporized. in this example, the expansion valve 24is controlled automatically by pressure which is the function of degreeof vaporization of the refrigerant fluid within the main evaporator 16.

The refrigerant is further vaporized within the evaporator 16 by heattransfer from hot wet compressed air supplied to the main evaporator 16through conduit 26. Between the end ofthe conduit 26 connected to theevaporator 16 and the inlet end 28 of the conduit 26, the regenerativeheat exchanger 14 is coupled. in this example, conduit 26 is effectivelyextended through the hut exchanger 14, which is formed in this exsm= plefrom s prlr of coaxial tubes 30. 32. The outer heat exchanger tube 30 iscoupled to conduit 34 forming the outlet of the main evaporator 16.Before reaching the main cvupora= tor 16, the pressurized and driedoutgoing air or other gas is therefore warmed regeneratively byheat-exchanging association with the hot wet compressed air passingthrough inner coaxial tube 32.

Within the main evaporator 16, water, lubricating oil, dirt and otherforeign matter are condensed or leached out of the pressurized gaspassing through the evaporator 16 by heatexchanging association with theexpanding refrigerant. Water or other foreign material is periodicallydrained from the evaporator 16 through drain valve 36.

The refrigerant, which is partially or completely vaporized in theevaporator 16 flows thence to the auxiliary evaporator 18, in thisexample, consisting of outer and inner coaxial tubes 38, 40. Suitableinstrumentation 42 can be coupled to outlet refrigerant conduit 44, bywhich the primary evaporator 16 is connected to the outer coaxial tube38 of the auxiliary evaporator 18. in passing through the auxiliaryevaporator 18, the output refrigerant from the primary evaporator 16 isexpanded further and returns to compressor 46 through conduit 48. Theexpansion of refrigerant in the auxiliary evaporator 18 in turn removesheat from refrigerant flowing from condenser 50 through conduit 52 whichis coupled to the inner tube 40 of the auxiliary evaporator 18 fromwhich it flows to receiver 20 through connecting conduit 56. Therefrigerant is then recycled from the receiver 20 to expansion valve 24and primary evaporator 16 through conduit 22.

in effect, then, both the incoming hot wet compressed air and therefrigerant supplied to the main evaporator 16 are cooled regenerativelybefore entering the primary evaporator 16. This arrangement permits thegas-handling equipment 10 to be controlled in the stable fashion under awide range of operation and ambient conditions. in particular,refrigerant is supplied to the condensing unit 50 (conduit 52) and tothe primary evaporator 16 (conduit 22) at correspondingly lowertemperatures, whereby a marked increase in efficiency is obtained.

The valve 24 itself is of conventional construction but is provided withan internal pressure-transmitting aperture 25 to render the valve 24sensitive to fluctuations in line pressure. Alternatively as denoted byconduit outline 27 the valve 24 can be sensitized through conduit 27 topressures elsewhere in the apparatus, for example the evaporator outletconduit 44. A suitable valve is available from Alco Controls Corp., St.Louis, Mo, for example catalog No. ACP-Z (internal pressuretransmitting), ACPE-S (external pressure transmitting) or GI -300. Othersizes are available depending on particular equipment capacities.Additionally, the valve 24 is provided with a stop member 29 to avoidcomplete closure of the valve 24.

in conventional gas-handling apparatus of this type, it is expected thata pressure-sensitive valve, such as the valve 24, will not closelyfollow load changes in the system, i.e., both the stability and thecontrol in the system will be poor, However, we attain an unexpectedlystable system having an excellent control characteristic by employingthe pressure-sensitive valve 24 in conjunction with a continuouslyoperated compressor 46. The compressor 46 is not operated intermittentlyor cyclically as in conventional systems. This eliminates one source ofthe phenomenon known as hunting in the system.

The use of a continuously operated compressor, such as the compressor 46in conjunction with the pressure-sensitive valve 24 lays a basis for thevastly improved control characteristic exhibited by our novel apparatus.As the valve 24 is prevented from complete closure, the compressor 46 issized so that it is onpuhlr of f cing he alve 24 m re near y c o whenrequir for ad changes in tho increasing direction. The valve 24 movestoward its open position upon oooroasing load, which increases load onthe auxiliary evaporator 18. While the main evaporator 16 loads, theauxiliary evaporator 18 unlosdo The compressor 46 also is solootod ontho oasis of known criteria, for oonstannspood operation rogurdiossottho load upon the system, within system capability. The use of oprouure sensitive valve, and a continuously operated oon= stant-speedcompressor, coupled with the ability of the compressor toadjust theexpansion valve opening endows the apparatus with considerable stabilityand with a control characteristic which is capable of compensating widevariations in incoming gas, pressure, temperature, and moisture content,as well as widely varying changes in ambient conditions. Our apparatus,therefore, is capable of supplying cool and dry compressed air or othergas at a consistent dewpoint dryness. At the same time, the cooperativeaspects ofthe pressure-sensitive valve 24 and the continuously operatedcompressor 46 positively prevent freezing of condensed moisture in anypart of the equipment.

As better shown in Figures 2-4 the package equipment unit is arrangedsuch that the refrigeration and condensing unit 12 is mounted within anappropriately shaped individual casing 58, while the primary evaporator16, auxiliary evaporator 18 and the regenerative eat exchanger 14 aresupported within a separate casing 60. Inlet and outlet conduits 28, 34respectively are supported upon a wall portion of the casing 60 t Figure2 of the drawings further illustrates a unique form of our primaryevaporator 16 which is useful in the operation of our novel gas handlingapparatus 10 or 10'. The evaporator 16 is arranged for obtaining anunusually high coefficient of heat transfer between air or otherpressurized gas entering and leaving the evaporator via conduits 26, 34respectively and the refrigerant or other heat exchange fluid enteringand leaving the evaporator 16 via conduits 22 and 44 respectively(Figure 4 v In this arrangement of the invention, the evaporator 16 isprovided with a heat-exchanging structure denoted generally by referencecharacter 62. The heat exchanger 62 in this ex ample includes aplurality of sheath tubes 64 each of which contains a spined inner tube66 supported substantially coaxially within the respective sheath tubes64. The convecting spines 68 of each tube 66 are made by lancing anderecting narrow strips of the wall material of the tubes 66, and theouter ends of the spines 68 are closely fitted within the as sociatedsheath or tube 64. The spines 68 are substantially less in thicknessthan that of the tube walls to avoid any possibility of leakage. Becausebases of the spines are rooted integrally in the tube walls, the heattransfer characteristics are excellent.

In the illustrated arrangement the heat exchanger 62 includes four suchsheath tubes 64 and a like number of associated or inner spined tubes66, although a different number of sheaths and tubes obviously can beutilized as required.

Two of the sheath tubes 64, for example 64a, and the associated coaxialtubes as better shown in Figure 5 are extended beyond the ends of theremaining sheath tubes 64b at the right end of the evaporator 16 asviewed in Figure 2 and are secured to baffle plate 70 where theycommunicate with plenum chamber 72 which in turn communicates with airinlet conduit 26.

The other ends of the sheath tubes 640 are secured to baffle plate 74adjacent the other end of the evaporator 16 and thus communicate withopposite end plenum chamber 76. Desirably, the remaining sheath tubes,such as the tubes 64b, likewise are secured to the right-hand baffleplate 74 for com munication with the adjacent plenum chamber 76 and withthe first group of sheath tubes 64a. The tubes 646 however terminateshort of the right hand baffle plate 70 at the opposite end of theevaporator 16 and are supported relative to one of the longer sheathtubes 640 by semicircular ring or bracket '78 (Figure 7) which desirablyis welded or otherwise secured to each of the shorter sheath tubes 64band to one of the longer sheath tubes 64a. Use of the bracket 78 doesnot interfere with the flow of fluid between and around the outersurfaces of the sheath tubes 64a, 64b the purpose of which flow isdescribed below. The flow baffle 74 at the opposite end of theevaporator 16 is provided with a central opening 80 as better shown inFigures 6 and 7 whereat the gas exit conduit 34 is Joined and sealed (asby welding or the like-Figure 2) for communication with fluid flowingbetween and around the sheath tubes 64.

With the urrungtu'nont just described, it will be seen that compressedair or other gas is caused to flow in parallel-series first through thelonger shouth tubes 64a and then through the shorter sheath tubes 64b.in addition, the compressed air or other fluid is induced to flowtransversely of the spines 68 to enhance heat transfer between therespective fluids within tubes 64, 66. More specifically, compressed airenters the lefthand plenum chamber 74, sis-viewed in FlCi. 4, from inletconduit 26 as denoted by flow arrow 64. From the chamber "is compressedair flows into both of the longer sheath tubes 64a as denoted by flowarrows 64 in FIG. 6. After following parallel annular paths through thesheath tubes 64o the compressed air enters plenum chamber '76 at: theopposite end of the evaporator 16. in the plenum chamber 76 thecompressed gas flows from the adjacent ends of the sheath tubes 64a tothe ends of the sheath tubes 64b as denoted by flow arrows 66 in FIG. 6.Thence, the compressed gas again follows parallel paths through thespines 68 of the shorter sheath tubes 646.

At the opposite ends of the shorter sheath tubes 646 the compressed gasenters a third plenum chamber 06 which is segregated from the adjacentend plenum chamber '73 by baffle 70. From the plenum chamber 66, asdenoted by flow arrows 90, the compressed gas flows between and aroundthe outer surfaces of the sheath tubes 64 to the central opening of thebaffle 74 at the opposite end of the evaporator 16. From the baffleopening 80, the compressed gas exits from the evaporator 16 throughconduit 34 as denoted by flow arrow 92. Alternatively, the exit conduit34 can be closed or the con duit 34 and the baffle opening 60 can beomitted, and the compressed gas can be withdrawn via chain-outlined exitconduit 94 which communicates with the plenum 66 through the spacesbetween and around the sheath tube 64 as denoted by flow arrows 96.

From the foregoing paragraph, it will be seen that pressure drops arereduced by causing the compressed gas to flow through successive pairsof the sheath tubes 64 in series-parallel fashion. The pressure drop ifdesired can be further minimized by flowing the gas through all of thesheath tubes in simple parallel fashion. 0n the other hand, therefrigerant fluid, because of lesser pressure drops, is flowed in seriesthrough the refrigerant tubes 66. Thus, the refrigerant fluid entersconduit 22 as denoted by arrow 98 and flows through refrigerant tube66a. At the other end of the refrigerant tube 66o, the fluid isconducted to refrigerant tube 66b through a reverse bend fitting or hosesection. 90. The refrigerant tube 666 at its opposite end is similarlycoupled by a second hose section 1100 to refrigerant tube 66c, which inturn is coupled to refrigerant tube 66d through a third hose fitting M3.The refrigerant tubes 66a and 66d are disposed in sheath tubes 64a whilerefrigerant tubes 66!; and 66c are in the shorter sheath tubes 64b. Thefourth refrigerant tube 66d communicates with exit refrigerant conduit44 as shown in FIG. 5. For convenience, the reverse fittings 98, 102,are disposed in plenum 76 while the reverse fitting 1106 is in the thirdplenum 66, of the evaporator R6. The flow of refrigerant successivelythrough the refrigerant tubes 66a, 66b, 66c and 66d and the respectivelyassociated fittings 98, 100, 102 is denoted respectively by flow arrows104, R06, 106.

It will be understood, of course, that other flow paths for thecompressed gas and the refrigerant fluid respectively can be providedthrough our novel heat exchanger structure 62. We have found howeverthat the aforedescribed flow paths are very effective in the efiicienttransfer of heat between com pressed gas and a refrigerant fluid. Itwill be further understood that other heat-exchanging fluids can beutilized in place of those noted above.

From the foregoing it will be apparent that novel and efficient forms ofcompressed air cleaners andlor dryers have been disclosed herein. Whilewe have shown and described certain presently preferred embodiments ofthe invention and have illustrated certain presently preferred methodsof practicing the same, it is o be distinctly understood that theinvention is not limited thereto but may be variously embodied andpracticed within the scope of be following claims.

We claim:

l. Gas-handling apparatus comprising a primary evaporator, first conduitconnections for establishing a flow path for a compressed gas and thelike through said evaporator, second conduit connections forestablishing a flow path of a refrigerant fluid through said primaryevaporator in heat exchanging relation with said gas path, apressure-sensitive expansion valve in said refrigerant path, arefrigerating unit coupied to said second conduit connections, anauxiliary evaporator coupled between said refrigerating unit and saidprimary evaporator, and pressure-sensitive means coupled to saidrefrigerant path adjacent its low-pressure side of the evaporator forcontrolling said expansion valve so that a variation in said compressedgas flow varies the flow of said refrigerant so as to divertcondensation thereof from said refrigerating unit to said auxiliaryevaporator .and/or to divert evaporation thereof from said mainevaporator to said auxiliary evaporatOl.

2. The combination according to claim 1 wherein said auxiliaryevaporator is coupled regeneratively to said second conduit connections.

3. The combination according to claim 1 wherein said refrigerating unitincludes a condensing unit therefor, some of said second conduitconnections couple said condensing unit to one side of said auxiliaryevaporator and others of said second conduit connections couple theother side of said auxiliary evaporator to said refrigerating unit andto said primary evaporator.

4. The combination according to claim 1 wherein a heat exchanger forsaid gas is coupled regeneratively to said first mentioned conduitconnections.

5. The combination according to claim 1 wherein said expansion valve anda third conduit connection are coupled between said primary evaporatorand said refrigerating unit in bypassing relation to said auxiliaryevaporator.

6. The combination according to claim 5 wherein said expansion valve isprovided with means for preventing its complete closure.

7. The combination according to claim 1 wherein said primary evaporatorincludes a heat exchanger structure having flow means associated withthe structure for inducing serpentine heat exchanging paths of said gasand of said refrigerant fluid therethrough.

8. Gas-handling apparatus comprising a primary evaporator, first conduitconnections for establishing a flow path for a compressed gas and thelike through said evaporator, second conduit connections forestablishing a flow path of a refrigerant fluid through said primaryevaporator in heatexchanging relation with said gas path, arefrigerating unit coupled to said second conduit connections, and anauxiliary evaporator coupled between said refrigerating unit and saidprimary evaporator, said primary evaporator including a heat exchangerstructure having flow means associated with the structure for inducingserpentine heat-exchanging paths of said gas and of said refrigerantfluid therethrough, said heatexchanging means including a plurality ofsheath tubes and inner tubes mounted spacedly within said sheath tubes,and flow means for conducting one of said gas and said fluidsequentially through one group of the group of sheath tubes and thegroup of inner tubes and for conducting the other of said gas and saidfluid in parallel through at least some of the remainder of said tubes.

9. The combination according to claim 8 wherein said inner tubes areeach provided with a plurality of heat transfer spines extendingtransversely between each coaxial tube and the as sociated sheath tube.

10. The combination according to claim 8 wherein said flow-conductingmeans are arranged to couple said inner tubes in series and said sheathtubes in series-parallel.

11. A method for operating refrigerating gas-handling apparatus having acompressor, condenser, main evaporator, auxiliary evaporator andexpansion valve, said valve being mounted for expansion of a refrigerantfluid into said main evaporator, said method comprising the steps ofoperating said compressor continuously, preventing complete closure ofsaid valve, sensitizing opening and closing movements of said valve topressure fluctuations in the path of said refrigerant fluid between therefrigerant outlet of said main evaporator and said auxiliaryevaporator, and operating said compressor at constant speed for forcingsaid valve further open in response to load changes.

12. The combination according to claim 1 wherein said ex pansion valveis coupled in an inlet portion of said refrigerant flow path relative tosaid primary evaporator, and said pres sure-sensitive means are coupledin an outlet portion of said refrigerant flow path relative to saidprimary evaporator.

13. The combination according to claim 12 wherein saidpressure-sensitive means are coupled between said primary evaporator andsaid auxiliary evaporator.

14. The combination according to claim 1 wherein said primary evaporatorincludes a heat exchanger structure having flow means therein forinducing a serpentine heatexchanging path in the flow of at least one ofsaid gas and of said refrigerant therethrough.

15. The combination according to claim 7 wherein said serpentineheat-exchanging paths are coaxial.

16. The combination according to claim 4 wherein auxiliary evaporatorand said heat exchanger are each shaped to define coaxialheat-exchanging paths therethrough.

2. The combination according to claim 1 wherein said auxIliaryevaporator is coupled regeneratively to said second conduit connections.3. The combination according to claim 1 wherein said refrigerating unitincludes a condensing unit therefor, some of said second conduitconnections couple said condensing unit to one side of said auxiliaryevaporator and others of said second conduit connections couple theother side of said auxiliary evaporator to said refrigerating unit andto said primary evaporator.
 4. The combination according to claim 1wherein a heat exchanger for said gas is coupled regeneratively to saidfirst-mentioned conduit connections.
 5. The combination according toclaim 1 wherein said expansion valve and a third conduit connection arecoupled between said primary evaporator and said refrigerating unit inbypassing relation to said auxiliary evaporator.
 6. The combinationaccording to claim 5 wherein said expansion valve is provided with meansfor preventing its complete closure.
 7. The combination according toclaim 1 wherein said primary evaporator includes a heat exchangerstructure having flow means associated with the structure for inducingserpentine heat exchanging paths of said gas and of said refrigerantfluid therethrough.
 8. Gas-handling apparatus comprising a primaryevaporator, first conduit connections for establishing a flow path for acompressed gas and the like through said evaporator, second conduitconnections for establishing a flow path of a refrigerant fluid throughsaid primary evaporator in heat-exchanging relation with said gas path,a refrigerating unit coupled to said second conduit connections, and anauxiliary evaporator coupled between said refrigerating unit and saidprimary evaporator, said primary evaporator including a heat exchangerstructure having flow means associated with the structure for inducingserpentine heat-exchanging paths of said gas and of said refrigerantfluid therethrough, said heat-exchanging means including a plurality ofsheath tubes and inner tubes mounted spacedly within said sheath tubes,and flow means for conducting one of said gas and said fluidsequentially through one group of the group of sheath tubes and thegroup of inner tubes and for conducting the other of said gas and saidfluid in parallel through at least some of the remainder of said tubes.9. The combination according to claim 8 wherein said inner tubes areeach provided with a plurality of heat transfer spines extendingtransversely between each coaxial tube and the associated sheath tube.10. The combination according to claim 8 wherein said flow-conductingmeans are arranged to couple said inner tubes in series and said sheathtubes in series-parallel.
 11. A method for operating refrigeratinggas-handling apparatus having a compressor, condenser, main evaporator,auxiliary evaporator and expansion valve, said valve being mounted forexpansion of a refrigerant fluid into said main evaporator, said methodcomprising the steps of operating said compressor continuously,preventing complete closure of said valve, sensitizing opening andclosing movements of said valve to pressure fluctuations in the path ofsaid refrigerant fluid between the refrigerant outlet of said mainevaporator and said auxiliary evaporator, and operating said compressorat constant speed for forcing said valve further open in response toload changes.
 12. The combination according to claim 1 wherein saidexpansion valve is coupled in an inlet portion of said refrigerant flowpath relative to said primary evaporator, and said pressure-sensitivemeans are coupled in an outlet portion of said refrigerant flow pathrelative to said primary evaporator.
 13. The combination according toclaim 12 wherein said pressure-sensitive means are coupled between saidprimary evaporator and said auxiliary evaporator.
 14. The combinationaccording to claim 1 wherein said primary evaporator includes a heatexchanger structure having flow means therein for inducing a serpentineheat-exchanging patH in the flow of at least one of said gas and of saidrefrigerant therethrough.
 15. The combination according to claim 7wherein said serpentine heat-exchanging paths are coaxial.
 16. Thecombination according to claim 4 wherein auxiliary evaporator and saidheat exchanger are each shaped to define coaxial heat-exchanging pathstherethrough.